U.S. Pat. Nos. 4,873,821; 5,003,763; and 5,012,636 describe a process apparatus and yarn product where alternate twist plied yarn is twisted in a first half-cycle of ply twist and is bonded while ply twisted and before the twist is reversed for the second half-cycle of ply twist. The yarn is bonded using an ultrasonic horn and anvil, the anvil having an elongated slot with a width slightly greater than the diameter of a single strand of yarn and a depth about equal to the combined diameters of the plurality of yarns ply-twisted together. The horn is a narrow rectangular blade shape with the thickness dimension being a close clearance fit with the width of the slot in the anvil; the clearance is slightly more than the diameter of an individual filament of the multifilament yarn strand. This arrangement produces a bonded portion of ply-twisted yarn that has a generally "U" shaped cross-section where the sides of the "U" have a small number of loosely gathered filaments and the bottom of the "U" has a large number of densely packed filaments. These patents teach that to make an ultrasonic bond between two or more strands of yarn, the strands should be essentially constrained around the circumference of the ply twisted strands, and compacted while applying ultrasonic energy. Adhesive joining occurs between all the compacted filaments. The loosely gathered filaments in the completed bond discussed above are only those that fit into the minimum clearance required between the closely fitting horn blade and anvil slot. The bonding tooling has the yarns constrained in the direction of motion of the moveable tool, and laterally, i.e. in a direction perpendicular to the direction of motion of the moveable tool.
The process and apparatus of the referenced patents produces a strong bond when the equipment is new and accurately adjusted. There is a problem during use, however, that the alignment between the closely fitting horn and anvil deteriorates and wear occurs. By contact with the sides of the anvil slot, the horn blade width may wear down 15%-30% in routine use which reduces the number of filaments compacted and the only filaments joined are those squeezed under the horn. The corners of the horn are sharpened by the wearing action so that the yarn filaments contacted by the sharp corners may be cut. This wear, then, leads to poor bonds. When the twist of the yarn strands is reversed adjacent a poor bond, the strand twisting force is sometimes strong enough that the poor bond is peeled apart. This is particularly true if the bond is still hot and softened from the bonding process. The poor bond may also have weakened filaments so that tensile forces used to advance the yarn are sufficient to break some of the strands at the bond, which may occur if the bond sticks momentarily in the narrow anvil slot or when the yarn is processed later. Broken strands or peeled apart bonds create processing problems and are not acceptable in the final alternate twist plied yarn product that is often used in a carpet structure. A "missed bond", where the strands are not held together adjacent to the twist reversal, permits the strands to un-ply for a distance of several inches which results in a fuzzy streak in a cut pile tufted carpet.
The bonds taught in the referenced patents also make a "stiff" segment in the yarn where the polymer has been softened and re-solidified throughout the bonded region; the adhesive joining in a good bond involves most of the filaments of the strands used. There is a need for a "softer" bond that has a higher fraction of loosely gathered filaments and is not noticeable when it appears in the tuft of a cut pile carpet. A stiff bond is also sometimes detrimental when the alternate twist plied yarn is used in a tufting machine to make carpet. Occasionally, the bond is in the eye of the tufting needle just as the needle is penetrating the primary carpet backing; the yarn at the ends of the bond is then subject to a high stress as the bond is forced through the backing. This may result in breaking of the yarn at the bond. U.S. Pat. No. 5,179,827 describes a soft bond, but there are very few filaments adhesively joined and the process is difficult to control, so the bond may fail as a result; or as the horn wears, a different bond is produced which is often stiff. Reliability is a problem since the process uses a close fitting horn and anvil configuration as just discussed.
There is a need for a more flexible bond that will bend when it is in the eye of the tufting needle just as the needle is penetrating the backing. Such a flexible bond may also have the benefit of being less obvious when it appears in a tuft of a cut pile carpet; it should compress and deflect and have loose filaments present to more closely resemble the cut un-bonded tufts.
In summary, there is a need for a bonding process that is less sensitive to wear and alignment of horn and anvil components and which will reliably produce a more robust bond that will withstand a greater peel force and tensile force without failure. There is a need for a bonded alternate twist plied yarn that has a more flexible bond and one that has a high fraction of loosely gathered filaments in the bond.